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Monolithic Slabs

We are often asked about doing shops or garages as monolithic slabs with haunched edges or footings. We can form them but do not recommend it. There are several drawbacks to the monolithic slab system when used in the Pacific Northwest.

Frost Code: Supporting footings must have between 12 and 18" of exterior backfill or "cover" to conform to frost code. This dictates that the thickened edge must be 16 to 24" tall or be set on a bed of free-draining gravel to achieve this depth.

Grades: For a conventionally framed building the framing and siding components must be set 3 to 6" above the exterior grade. This necessitates that the grade of the interior slab be 3 to 6" higher than the surrounding grade. In other words, the floor is higher than the lawn. For a building with drive-in access like a shop or garage this means that you must have ramps at all exterior garage doors and steps at exterior man doors.

Cracking: Concrete moves over time; it shrinks incrementally for up to three months after placement and thereafter expands and contracts with temperature changes. A slab with "heavyups" like thickened edges, footings, or piers will crack along every change in thickness. See drawing below.

Cost: Cost in forming, excavating the haunched edges, and the increased use of material.

It is our opinion, based on many situations over 30 years, that it is most often not only better to pour a stem wall with a separated interior slab, but that it is less expensive!